For me the history, evolution, and fundamental purpose behind the city is what's most important. So often I see fantasy cities that seem to have poofed into existence fully formed.

Why did a population choose to gather together in that place at all? How did their settlement grow over time? What resources did they have available to them during the history, both in terms of building material, architectural expertise, and, in a fantasy setting, magic? All these things determine how a city got to be where it is. They are the path along which it and its populace has tread.

Take, for example, Oxford. The name says it all. It was originally a ford on the river Thames where oxen could cross. This caused it to become a key post along a limited trade route. Read a little of its history and you understand it has a rich past with the church, military, and so on, all of which have resulted in the city we have today.

I take the greatest pleasure in experiencing cities (and their accompanying maps) that have a similar level of depth and verisimilitude.